The invention relates to an air-brake line pipe which comprises at least one polyamide layer and at least one polypropylene layer.
Air-brake systems are often used in heavy-duty vehicles, e.g. in tractor vehicles and the like. In systems of this type, the braking system is activated via compressed air, which is passed through a pipe.
Air-brake lines are currently mainly manufactured from single-layer PA11 pipes or from single-layer PA12 pipes; there are moreover multilayer solutions in which the intention is to achieve high bursting pressure with good flexibility, by means of textile reinforcement. In regions where the level of mechanical and chemical requirements is very low, systems based on polyurethane are mainly used. Particular mention may be made here of the market for semitrailers and the market for replacement parts. These systems are less expensive than systems based on PA11 or on PA12, but have marked disadvantages in mechanical strength and in chemical resistance.
Higher polyamides, such as PA612, PA11, or PA12 are resistant to cracking, and resistant to attack by engine fuels, and by oils and water. Furthermore, they are unlike PA6 in being resistant to stress-cracking via zinc chloride. However, they are relatively expensive. Intensive work is being carried out on inexpensive alternatives, in order to comply with the price pressure present in the market.
DE-A 37 15 251 discloses a two-layer pipe which can be used as air-brake line. It is composed of a thick outer layer composed of polyamide, such as PA12, and of a thin inner layer composed of a polyolefin, such as maleic-anhydride-grafted polypropylene, which is intended to serve as barrier with respect to the alcohol present in the pressurized fluid. In a further development of this, the pipe of DE-C 38 21 723 also comprises an internal protective layer composed of a polyamide. However, pipes of this type achieve no cost advantage over single-layer polyamide pipes.
WO 01/64436 describes a multilayer pipe with an inner polyamide layer; the middle layer and the outer layer are composed of relatively precisely specified polypropylene molding compositions. That pipe is intended to be suitable for engine-fuel-line systems, pressure lines, and other lines in automobile construction. However, pipes with polypropylene outer layer are criticized inter alia on grounds of inadequate aging resistance of the polypropylene.
DE-A 101 37 863 describes another technical solution for an air-brake line; it is composed of a pipe whose layer sequence is PA11 or PA12/adhesion promoter/impact-modified PA6 or PA66; if desired, there are also the following adjoining layers: another adhesion-promoter layer, and a final layer composed of PA11 or PA12. As an alternative to this, the layer sequence of the pipe of DE-A 101 37 863 can be: PA612/impact-modified PA6 or PA66/PA612. Since the layer of PA6 or of PA66 is present with an unprotected face side, the insufficient resistance to zinc chloride in the systems has to be regarded as critical.